U.S. patent application number 12/349059 was filed with the patent office on 2009-07-16 for apparatus and method for inspection and measurement.
Invention is credited to Zhaohui CHENG, Natsuki TSUNO.
Application Number | 20090179151 12/349059 |
Document ID | / |
Family ID | 40849835 |
Filed Date | 2009-07-16 |
United States Patent
Application |
20090179151 |
Kind Code |
A1 |
CHENG; Zhaohui ; et
al. |
July 16, 2009 |
APPARATUS AND METHOD FOR INSPECTION AND MEASUREMENT
Abstract
An electrification control electrode B is installed at a
measured or inspected specimen side of an electrification control
electrode A, and a constant voltage is applied from an
electrification control electrode control portion of an
electrification control electrode B according to an electrification
state of a specimen, whereby a variation of an electrification
state and a potential barrier of a specimen surface formed before
an inspection is suppressed. A retarding potential is applied by an
electrification control electrode, and the electrification control
electrode B is disposed below the electrification control electrode
A adjusted to equal potential to a specimen. As a result, it is
possible to adjust the amount that secondary electrons emitted from
a specimen such as a wafer to which a primary electron beam is
irradiated return to a specimen, and thus it is possible to stably
maintain an inspection condition of high sensitivity during an
inspection.
Inventors: |
CHENG; Zhaohui; (Tokyo,
JP) ; TSUNO; Natsuki; (Kunitachi, JP) |
Correspondence
Address: |
MATTINGLY & MALUR, P.C.
1800 DIAGONAL ROAD, SUITE 370
ALEXANDRIA
VA
22314
US
|
Family ID: |
40849835 |
Appl. No.: |
12/349059 |
Filed: |
January 6, 2009 |
Current U.S.
Class: |
250/307 ;
250/310 |
Current CPC
Class: |
H01J 2237/24475
20130101; H01J 2237/2448 20130101; H01J 37/244 20130101; H01J
2237/2817 20130101; H01J 2237/24592 20130101; H01J 2237/24564
20130101; H01J 37/28 20130101 |
Class at
Publication: |
250/307 ;
250/310 |
International
Class: |
G01N 23/00 20060101
G01N023/00; G21K 7/00 20060101 G21K007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 11, 2008 |
JP |
2008-004407 |
Claims
1. An apparatus for inspection and measurement which scans a
semiconductor device having a pattern portion as an inspection
target and an insulator portion formed around the pattern portion
by a primary electron beam, detects a secondary signal of either or
both of a secondary electron and a reflected electron generated
from the semiconductor device, and images and displays the
secondary signal detected, the apparatus comprising: an object lens
for converging the primary electron beam; an electronic optical
system which includes a ground electrode, an electrification
control electrode A to which retarding potential is applied, and an
electrification control electrode B which are disposed in order
toward a semiconductor device side in a space above the
semiconductor device; and a power source control portion which
applies predetermined potential to the electrification control
electrode A, the electrification control electrode B and the
semiconductor device, respectively.
2. The apparatus of claim 1, wherein the power source control
portion set applied potentials of the electrification control
electrode A and the electrification control electrode B so that
minimum potential between the electrification control electrode A
and the electrification control electrode B is greater than minimum
potential between the semiconductor device and the electrification
control electrode B.
3. The apparatus of claim 1, further comprising: a measuring
portion which measures electric potential of the pattern portion
and the insulator portion of the semiconductor device, wherein the
power source control portion controls the applied potential and
sets the semiconductor device to an electrification state by using
a measurement value of the measuring portion.
4. The apparatus of claim 3, wherein the power source control
portion controls the applied potential based on a condition
selected according to electrical characteristics of the pattern
portion and the insulator portion and a purpose of an
inspection/measurement so that a potential difference between the
pattern portion and the insulator is smaller.
5. The apparatus of claim 3, wherein the power source control
portion adjusts at least one of voltages which are applied to the
electrification control electrode A, the electrification control
electrode B and the semiconductor device according to an
electrification state of the semiconductor device.
6. An apparatus for inspection and measurement which scans a
specimen having a pattern portion as an inspection target and an
insulator portion formed around the pattern portion by a charged
particle beam, detects a secondary signal resulting from a
secondary particle generated the specimen, and images and displays
the secondary signal detected, the apparatus comprising: a charged
particle beam source for generating the charged particle beam; a
detector for detecting the secondary particle and generating the
secondary signal; an image processing portion for processing the
secondary signal from the detector; a scanning deflector for
scanning the charged particle beam; an object lens for converging
the charged particle beam; an electronic optical system which
includes a ground electrode, a first electrification control
electrode to which retarding potential is applied, and a second
electrification control electrode which are disposed in order
toward the specimen between the object lens and the specimen; and a
power source control portion which applies predetermined potential
to the first electrification control electrode and the second
electrification control electrode of the optical system and the
specimen, respectively.
7. The apparatus of claim 6, wherein the power source control
portion controls applied potentials applied to the first
electrification control electrode and the second electrification
control electrode so that minimum potential between the first
electrification control electrode and the second electrification
control electrode is greater than minimum potential between the
specimen and the second electrification control electrode.
8. The apparatus of claim 7, further comprising: a measuring
portion which measures electric potential of the pattern portion
and the insulator portion of the specimen, wherein the power source
control portion controls the applied potential applied to the first
electrification control electrode and the second electrification
control electrode by using a measurement value of the measuring
portion.
9. The apparatus of claim 8, wherein the power source control
portion controls the applied potential applied to the first
electrification control electrode and the second electrification
control electrode so that a potential difference between the
pattern portion and the insulator is smaller.
10. The apparatus of claim 8, wherein the power source control
portion adjusts at least one of voltages which are applied to the
first electrification control electrode, the second electrification
control electrode and the specimen according to an electrification
state of the specimen.
11. A method for inspection and measurement of a semiconductor
device having a pattern portion as an inspection object and an
insulator portion formed around the pattern portion, comprising:
scanning the semiconductor device by a primary electron beam
converged by an object lens; controlling electrification potential
on a surface of the semiconductor device by using an electronic
optical system which includes an electrification control electrode
A to which retarding potential for the semiconductor device is
applied and an electrification control electrode B disposed toward
a semiconductor device side from the electrification control
electrode A in a space above the semiconductor device; detecting a
secondary signal of either or both of a secondary electron and a
reflected electron generated from the semiconductor device by
irradiation of the primary electron beam; and imaging and
displaying the detected signal.
12. The method of claim 11, further comprising: measuring
electrical potential of the pattern portion and the insulator
portion; and setting applied potentials of the electrification
control electrode A and the electrification control electrode B so
that minimum potential between the electrification control
electrode A and the electrification control electrode B is greater
than minimum potential between the semiconductor device and the
electrification control electrode B.
13. The method of claim 11, further comprising: selecting a
condition for electrically charging the semiconductor device
according to a distribution and individual electrical
characteristics of the pattern portion and the insulator portion
and a purpose of an inspection/measurement so that a potential
difference between the pattern portion and the insulator is as
small as possible.
14. The method of claim 11, further comprising: adjusting at least
one of voltages which are applied to the electrification control
electrode A, the electrification control electrode B and the
semiconductor device according to an electrification state of the
semiconductor device.
15. The method of claim 14, wherein a surface of the semiconductor
device is charged to positive electrification by the adjusting
step.
Description
CLAIM OF PRIORITY
[0001] The present application claims priority from Japanese patent
application JP 2008-004407 filed on Jan. 11, 2008, the content of
which is hereby incorporated by reference into this
application.
FIELD OF THE INVENTION
[0002] The present invention relates to a technology for
measuring/inspecting a fine circuit pattern formed on a substrate
of, for example, a semiconductor device or a liquid crystal by
using a charged particle beam, and more particularly, to a charged
particle beam apparatus which controls an electrification state of
a specimen surface to perform the measurement/inspection.
BACKGROUND OF THE INVENTION
[0003] A semiconductor device is manufactured by repetitively
performing a process for transferring a pattern formed on a photo
mask onto a wafer through a lithography process and an etching
process. In a semiconductor manufacturing device, since a yield of
a semiconductor device is influenced by a failure in a
manufacturing process such as an etching process and generation of
alien substances, it is important to inspect/measure a pattern on
wafer during a manufacturing process to detect an occurrence of an
abnormality or a failure as early as possible. Therefore, in a
current semiconductor device manufacturing line, a technique for
inspecting/measuring a state of a pattern formed on a wafer during
a manufacturing process plays an important role. Conventionally, an
inspection/measurement technique is mostly based on an optical
microscope, but recently an inspection/measurement apparatus based
on an electronic microscope is widely being spread to cope with
miniaturization of a semiconductor device and sophistication of a
manufacturing process. Particularly, in managing a dimension of a
semiconductor circuit pattern, a length measuring scanning electron
microscope (SEM) based on an electronic microscope is currently
used as a quality managing means which is indispensable to a
manufacturing process. In managing a dimension of a fine pattern,
high surface resolution, high measuring accuracy, and high
reproducibility are required, and it is also indispensable to
suppress damage to a circuit pattern when measured. In order to
satisfy such requirements, a primary electron beam is accelerated
at high energy and is decelerated, before being incident to a
specimen, at a retarding voltage applied to a specimen containing a
semiconductor pattern which is a measurement target.
[0004] However, if a surface of a semiconductor device containing
an insulator is scanned by a primary electron beam, an
electrification state of a surface may change depending on a
scanning condition. Therefore, the following faults may occur: (1)
a detection rate of a secondary signal emitted from a pattern
portion fluctuates, and an abnormal contrast occurs in a secondary
signal image; and (2) a scanning position of a primary electron
changes depending on a change of electrification, and measurement
accuracy and reproducibility of a pattern dimension may
deteriorate. Therefore, it is important to detect an
electrification state of a semiconductor device and to feed it back
to a measurement condition before measurement and to maintain an
electrification state of a semiconductor device surface during
measurement.
[0005] Also, in inspecting a semiconductor device, it is highly
required to detect an electrical characteristic fault such as
conduction and non-conduction which an optical inspection apparatus
is difficult to detect, and thus an electron beam inspection
apparatus comes into wide use. An electron beam inspection
apparatus detects an electrical characteristic fault by charging a
circuit pattern formed on a wafer surface and using a contrast
actualized by it. It is called a potential contrast technique, and
it is a useful means to detect an electrical characteristic fault
of a semiconductor device. In order to detect such a fault with
higher sensitivity, it is indispensable to appropriately charge a
semiconductor device.
[0006] As a technique for controlling an electrification state of
an inspected/measured specimen at high accuracy, Japanese Patent
Laid-open Publication no. 2000-208579 discloses a technique that a
desired voltage is applied to an electrode called an
electrification control electrode disposed opposite to a specimen,
and an electron beam is irradiated to a specimen from a secondary
electron source, which is different from an electron source for a
primary electron beam, to control electrification potential of a
specimen. PCT Publication no. WO2003/7330 discloses a technique
that surface potential is measured by using a surface potential
meter (SPM), and a preliminary electrification/destaticization
condition or an inspection/measurement condition of a semiconductor
device surface is optimized based on the result.
[0007] A principle of controlling electric potential of a specimen
surface using an electrification control electrode will be
described with reference to FIG. 4. FIG. 4 is a diagram
illustrating a disposition relationship between an inspected
specimen and an electrification control electrode when a contact
hole having a conduction defect formed therein is used as an
inspection specimen. An inspected specimen has a structure in which
a SiO.sub.2 layer 405 is formed on a Si substrate 404, a contact
hole is formed, and metal is embedded inside the hole as shown in a
cross-sectional view of a wafer 400 of FIG. 5.
[0008] An electron source 10 and an electrification control
electrode 407 are disposed above the wafer 400. The electrification
control electrode 407 has a hole which a primary electron beam and
a secondary charged particle pass through. Various lenses are
disposed between the electron source 10 and the electrification
control electrode 407 but are not shown in FIG. 4. A reference
numeral 17 denotes a reflecting plate 17, and a reference numeral
411 denotes a secondary electron detector. Retarding potential 406
is applied to the wafer 400, and predetermined potential
(electrification control electrode potential) 408 based on the
wafer 400 is applied to the electrification control electrode 407.
The primary electron beam 410 arriving at the wafer interacts with
the wafer to generate the secondary charged particle.
[0009] In a potential contrast technique, a difference between a
normal portion and a defective portion is detected as a contrast
difference of a potential contrast image. A contrast difference
results from the fact that an electrification potential difference
occurs since a normal portion and a defective portion are different
in electric resistance, and as a result, there occurs a difference
in number of secondary electrons detected. Therefore, in order to
detect a fault by a potential contrast technique, there is a need
for electrically charging a wafer to make a sufficient
electrification potential difference between a normal portion and a
defective portion. A wafer surface can be electrically charged to
either of (1) a positive voltage contrast (PCV) and (2) a negative
voltage contrast (NVC), and a polarity of electrification depends
on a structure of a wafer which is an inspection target or an
inspection condition. Here, a principle of a wafer negative voltage
contrast (NVC) will be described below.
[0010] A potential distribution is formed between the
electrification control electrode 407 and the wafer 400 by electric
potential 408 of the electrification control electrode 407 and
electric potential 406 of the wafer 400. A change of a potential
distribution along an optical axis of a primary electron beam is
indicated by a curve 413 of FIG. 4. As indicated by the curve 413,
in the potential distribution, there exists a position where
electric potential is minimum (position where electric potential
becomes negative maximum), and a potential difference 412 between
electric potential at the position (hereinafter, minimum potential)
and wafer surface potential functions as a potential barrier of a
secondary signal emitted from a wafer surface.
[0011] In the secondary signal 409 emitted from the wafer 400 by
irradiation of the primary electron beam, an element that kinetic
energy is higher than the potential barrier 412 goes over the
barrier and is detected by the detector 411. Meanwhile, an element
of the secondary signal that kinetic energy is lower than the
potential barrier 412 returns to the wafer surface 414 and
electrically charges the wafer to a negative. In order to
electrically charge the wafer to a positive, a voltage applied to
the electrification control electrode 407 is appropriately adjusted
so that the number of secondary electrons emitted from the wafer
can be greater than the number of electrons contained in the
primary electron beam which arrives at a specimen. As a result, the
wafer surface is electrically charged to a positive.
[0012] U.S. Pat. No. 6,586,736 B1 discloses an invention which
applies an electrification control electrode described above.
According to an invention disclosed in U.S. Pat. No. 6,586,736 B1,
if an incident angle of a primary electron beam to a specimen is
deflected (strays) from an electron beam optical axis, secondary
charged particles which return to a specimen surface are increased,
so that it is difficult to control a potential distribution of a
specimen surface. In order to resolve the problem that an incident
angle of a primary electron beam is deflected, U.S. Pat. No.
6,586,736 B1 employs a three-electrode structure as an
electrification control electrode, sets an electrode (i.e., lowest
electrode) proximal to a specimen to the same voltage as a
retarding voltage, divides an intermediate electrode into left and
right centering on an optical axis, and changes voltages applied to
the divided electrodes left and right.
SUMMARY OF THE INVENTION
[0013] A problem in controlling an electrification control
electrode in a conventional potential contrast technique will be
described from a point of view of a change of a potential
barrier.
[0014] FIG. 5A is an enlarged view illustrating a structure around
the electrification control electrode shown in FIG. 4. If
predetermined electric potential is applied to the electrification
control electrode 407, a potential distribution of a concentric
circle shape centering on an optical axis of the primary electron
beam 410 is formed. Such a potential distribution centering on an
optical axis of the primary electron beam 410 is also called
on-axial potential. In FIG. 5, a reference numeral 413 denotes an
equipotential line which represents a cross section of a certain
equipotential surface of on-axis potential. Meanwhile, a potential
distribution is formed even in an area which deviates from an
optical axis of the primary electron beam 410 due to electrical
potential applied to the electrification control electrode 407.
Such a potential distribution formed around the primary electron
beam optical axis is also called off-axis potential. In FIG. 5, a
reference numeral 418 denotes an equipotential line which
represents a cross section of a certain equipotential surface
having off-axis potential.
[0015] However, as described above, in a potential contrast
technique, a difference between a normal portion and a defective
portion is detected as a contrast difference of a potential
contrast image. In case where a specimen is inspected by
electrically charging a defective portion to a more negative than a
normal portion, since the number of secondary electrons which go
over the potential barrier 412 is more in a defective portion than
in a normal portion, it is more brightly seen on a potential
contrast image. Such a brightness difference (contrast) between a
normal portion and a defective portion is determined by an
electrification state of a defective portion/normal portion and the
depth of the potential barrier 412. In order to realize high
stability and high reproducibility, it is necessary to maintain an
optimum electrification state by constantly keeping the potential
barrier 412 during an inspection.
[0016] FIG. 5B shows a comparison of energy distributions of
secondary electrons respectively emitted from the defective portion
402 and the normal portion 401. It is understood that compared to
an energy distribution of secondary electrons emitted from the
normal portion, an energy distribution of secondary electrons
emitted from the defective portion contains more elements that
energy is higher than the potential barrier 412. Therefore, a
contrast difference of an electron beam image can be controlled by
controlling the size of the potential barrier 412.
[0017] In the electrification control system of the structure shown
in FIG. 5A, electric potential applied to the electrification
control electrode 407 depends on a different between a target value
of electrical potential of the inspected specimen 400 and
electrical potential applied to the specimen 400. That is, a
difference between the retarding potential 406 and the control
target value of the specimen potential is applied to the
electrification control electrode 407. As a result of an
experiment, however, it is turned out that a potential barrier
which is deeper than the potential barrier 412 defined by the
on-axis potential is formed between the wafer surface 414 and the
electrode 407 in a direction apart from an optical axis due to the
set potential 408 of the electrification control electrode 407.
[0018] Here, a term "electrical control system" is used as a
concept containing other elements such as a control unit, a control
power source, and so on as well as hardware such as an
electrification control electrode. Since secondary electrons
generated by irradiation of a primary electron beam are
cosine-distributed by energy at the moment when they are generated,
they are emitted in an off-axis direction as well as in a direction
proximal to an optical axis. The emitted secondary electrons 428
are returned to the specimen surface by an off-axis potential
barrier to be redirected toward the set potential 408 of the
electrification control electrode 407 through the contact hole and
the wafer.
[0019] For this reason, the potential barrier 412 formed between
the specimen and the electrification control electrode varies from
an intended value, and the acquired contrast of the potential
contrast image varies from an intended value. As a result,
inspection sensitivity becomes unstable, and inspection
reproducibility also deteriorates.
[0020] Next, a problem of the electrification control system
disclosed in U.S. Pat. No. 6,586,736 B1 will be described. In the
electrification control system disclosed in U.S. Pat. No. 6,586,736
B1, the electrification control electrode includes electrodes 431,
432 and 433. During an inspection (i.e., when image data are
acquired by irradiating a primary electron beam), the same voltage
as a voltage 406 applied to a specimen holder on which an inspected
specimen is placed, i.e., the retarding potential, is applied to
the electrode 431, and electrification formed on a semiconductor
device surface is suppressed during an inspection. The electrode
433 remains at ground potential. The electrode 432 is divided into
two (432a and 432b) to which different voltages 435 and 436 are
applied. It is an object to converge an orbit of secondary
electrons emitted to an area except an electron optical axis when
an electron beam is irradiated to the semiconductor device 400,
thereby improving detection efficiency of secondary electrons
(e.g., secondary signal 437). However, before an inspection, in
order to obtain a higher contrast, pre-charging is mostly performed
by irradiating a charged particle beam onto a semiconductor device
surface. As a result, electrification potential of a semiconductor
device surface changes, so that a potential distribution between
the semiconductor device surface 414 and the electrode 431 slightly
change, and even though an electron beam is irradiated under the
same optical condition as an irradiation condition of an electron
beam set before pre-charging, there is a possibility that a
detection rate of a secondary signal or electrification of a
semiconductor device surface changes during an inspection.
[0021] Also, since a pattern is formed on a semiconductor device
surface, when acquiring an image while scanning an electron beam,
an electron beam may be irradiated to areas (e.g., area 414a and
area 414b) which are different in electrification characteristic.
If an electrification characteristic changes electrification
potential which can be consequently acquired changes, and thus an
orbit of secondary electrons emitted from each area changes
according to an electrification state of each area. This will be
described with reference to FIG. 5D. An area 443 is an area which
has hole patterns massed on a semiconductor device, and an area 444
represents an area around which a pattern does not exist. An
electron beam is scanned up and down along an arrow shown in FIG.
5D. Since the specimen 400 moves in a horizontal direction due to a
continuous stage movement, an electron beam is irradiated in order
of, for example, the area 444.fwdarw.the area 443.fwdarw.the area
444 within an upper-lower scanning range. In FIG. 5D, a solid line
arrow means that an electron beam arrives at a specimen, and a
dotted line means that an electron beam does not arrive at a
specimen due to, for example, blanking.
[0022] In case where a scanning area of the electron beam 19
overlaps the areas 443 and 444 which are different in
electrification characteristic as described above, different
electrification states are formed in the two areas, so that
electric field of a horizontal direction is formed between the two
areas. As a result, secondary electrons generated within the area
443 are bent in a constant direction. In the electrode structure
shown in FIG. 5C, if voltages of the electrodes 432a and 432b are
adjusted according to an orbit of secondary electrons emitted from,
for example, a position 441, effective detection can be performed
with respect to secondary electrons emitted from the position 441
(e.g., secondary signal 437). However, in secondary electrons
emitted from a scanning position 442 which is an opposite end to
the area 443 (the position 441), secondary electrons are bent in
the same direction, and thus detection efficiency of a secondary
signal gets worse under a voltage set condition of the electrodes
432a and 432b (e.g., secondary signal 438). Also, since division of
the electrode influences the beam size or an orbit of an electron
beam for inspection, deterioration of resolution or scanning
transformation is unavoidable.
[0023] In a current situation, an effective means for accurately
measuring an electrification state of a very small area such as a
scanning area of an electron beam does not exist. Therefore,
control for monitoring an electrification state of an irradiation
area of an electron beam and feeding it back to a voltage applied
to the electrode 432 is impossible in a current situation, and
control of an applied voltage value of the electrification control
electrode cannot help relying on an experience principle.
Therefore, it takes a lot of time to find an optimum forming
condition of an electrification state or an appropriate inspection
condition for defective contrast emphasis, and even if an optimum
forming condition of an electrification state is found, the forming
condition is not always a condition for maintaining a stable
electrification state during an inspection. As described above, the
convention measurement or inspection apparatus has a problem in
that it is difficult to perform an inspection of both high
sensitivity and high stability.
[0024] As described above, a brightness difference (contrast)
between a normal portion and a defective portion depends on an
electrification state of a defective portion/normal portion and the
depth of the potential barrier 412. In the present invention, an
electrification control electrode B is installed below a
conventional electrification control electrode A (at a measured or
inspected specimen side), and a constant voltage is applied
according to an electrification state of a specimen, whereby a
fluctuation of an electrification state and a potential barrier of
a specimen surface formed before an inspection is suppressed. Due
to an electrode disposed below an electrode adjusted to equal
potential to a specimen, it is possible to adjust the amount that
secondary electrons emitted from a specimen return to a specimen,
and it is also possible to stably maintain an inspection condition
of high sensitivity during an inspection. Here, "conventional
electrification control electrode" means an electrode to which
equal retarding potential to a specimen can be applied and
corresponds to an electrode 431 in the conventional electrode
structure shown in FIG. 5C.
[0025] Various potentials are applied to the electrification
control electrodes A and B related to the present invention
depending on an electrification state formed on a specimen. For
example, if raising electric field which prevents secondary
electrons from returning to a specimen is formed between a specimen
and an electrification control electrode, a specimen surface can be
charged to a PVC. If deceleration electric field which has
secondary electrons to return to a specimen is formed, a specimen
surface is charged to a NVC. Also, in this specification, the
electrification electrodes A and B may be called first and second
electrification control electrodes, respectively.
[0026] In order to form an appropriate electrification state in an
inspection area, an inspection/measurement apparatus of the present
invention employs a means for measuring electrification of a
semiconductor device to grasp an electrical characteristic of an
area which is an inspection/measurement target. As a result, a set
condition of an electrification/destaticization means can be
optimized, whereby a proper electrification state can be formed for
an inspection/measurement.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a schematic diagram illustrating a scanning type
inspection/measurement apparatus of a regarding method according to
a first embodiment of the present invention;
[0028] FIG. 2 is a view illustrating an inspection/measurement
sequence according the first embodiment of the present
invention;
[0029] FIG. 3A shows a measurement result according to a second
embodiment of the present invention;
[0030] FIG. 3B shows correlation data between a
pre-charging/destaticizing set condition and electrification
potential of respective areas on a semiconductor device;
[0031] FIG. 4 is a view illustrating an SEM image of the
semiconductor device;
[0032] FIG. 5A is an enlarged view illustrating a structure around
the electrification control electrode shown in FIG. 4;
[0033] FIG. 5B shows a comparison of energy distributions of
secondary electrons respectively emitted from the defective portion
and the normal portion;
[0034] FIG. 5C shows a structure of a conventional electrification
control electrode;
[0035] FIG. 5D shows an orbit change of secondary electrons emitted
from each area;
[0036] FIG. 6 is a view illustrating an electrification control
electrode structure which achieves both contrast optimization and
electrification stabilization during an inspection/measurement
according to the present invention;
[0037] FIG. 7 is a view illustrating an inspection/measurement
sequence using correlation data of pre-charging shown in FIG. 3
according to a second embodiment of the present invention;
[0038] FIG. 8A is a view illustrating a principle for measuring
electrification potential of a semiconductor device according to a
third embodiment of the present invention; and
[0039] FIG. 8B is a view illustrating a sequence for measuring
electrification potential of a semiconductor device according to a
third embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] Hereinafter, an inspection method and apparatus according to
embodiments of the present invention will be described in detail
with reference to drawings, and a basic configuration of the
present invention will be first described with reference to FIG.
6.
[0041] As shown in FIG. 6, an electrification control electrode
B421 is installed below a conventional electrification control
electrode A420, that is, at a measured or inspected specimen side,
and a constant voltage is applied from a control power source 422
according to an electrification state of a semiconductor device 400
which is a specimen, whereby a fluctuation of an electrification
state of a specimen surface and a potential barrier formed before
an inspection is suppressed. Since the electrification control
electrode B421 is disposed below the electrification control
electrode A420 adjusted to equal potential to a specimen, it is
possible to adjust the amount that secondary electrons emitted from
a specimen return to a specimen, and it is also possible to stably
maintain an inspection condition of high sensitivity during an
inspection.
[0042] Also, as shown in FIG. 6, the amount which returns to the
semiconductor device among the second signals 409 generated by
irradiation of the primary electron beam 410 and the returning
position thereof can be controlled by adjusting a voltage applied
to the electrification control electrode B421 by the control power
source 422 according to an electrification state of the
semiconductor device 400, whereby it is possible to constantly
maintain an electrification state of the semiconductor device 400
during an inspection/measurement. Since a potential difference
between the electrification control electrode and the semiconductor
device can be controlled within a constant range, a potential
barrier can always remain constant during an
inspection/measurement, thereby realizing an inspection/measurement
of high sensitivity while keeping high reproducibility.
First Embodiment
[0043] FIG. 1 shows a configuration of an inspection apparatus
according to a first embodiment of the present invention. The
inspection apparatus according to the first embodiment of the
present invention is a scanning electron microscopy (SEM) which
includes a means for measuring surface potential of a specimen and
an electrification control means and may be applied to an
inspection SEM, a review SEM, and a length measuring SEM.
[0044] The SEM of FIG. 1 includes a chamber 2 that the inside
thereof is vacuum-exhausted and a specimen exchange chamber 62
which functions as a preliminary chamber for carrying a wafer 9 as
a specimen into the chamber 2, and the preliminary chamber is
configured to be vacuum-exhausted independent of the chamber 2. The
inspection apparatus further includes a control portion 6 and an
image processing portion 5 in addition to the chamber 2 and the
preliminary chamber. The control portion 6 has a function of an
apparatus controller for controlling the whole operation of the
inspection apparatus, and a general-purpose computer having a
central processing unit (CPU) may be used as the control portion
6.
[0045] The chamber 2 includes an electronic optical system 3, an
electrification control portion which will be described later in
detail, a detecting portion 7, a specimen chamber 8, and an optical
microscope portion 4. In the first embodiment of the present
invention, the chamber 2 means the whole vacuum vessel including
the specimen chamber 8, and the electronic optical system 3, the
electrification control portion, the detecting portion 7, and the
optical microscope 4 operate in a decompressed state within the
vacuum vessel. The specimen chamber 8 is a concept representing a
space in which a specimen stage is driven within the chamber 2, and
an area defined by a dotted line of FIG. 1 corresponds to the
specimen chamber. As an inspected specimen, there is a
semiconductor wafer on which a wire line pattern or a circuit
pattern is formed, a specimen piece cut from a wafer, or a
semiconductor chip having a circuit formed therein, but it is
possible to observe electric potential of a specimen such as a
magnetic head, a recording medium or a liquid crystal panel besides
a semiconductor device.
[0046] The electronic optical system 3 includes an electron source
10, an electron beam extracting electrode 11, a condenser lens 12,
a blanking deflector 13, a scanning deflector 15, an iris 14, an
object lens 16, a secondary signal converging lens 69, a reflecting
plate 17, and an E.times.B deflector 18. In the detecting portion
7, a detector 20 is disposed above the object lens within the
chamber 2. An output signal of the detector 20 is amplified by a
pre-amp 21 installed outside the chamber 2 and is converted into
digital data by an AD converter 22.
[0047] The electrification control portion includes an
electrification control electrode, an electrification control
electrode control portion, and an electrification control power
source, and in the configuration of FIG. 1, it includes
electrification control electrodes A420 and B421 installed opposite
to a stage, electrification control electrode control portions 66
and 423, and power sources 67 and 424. In FIG. 1, the
electrification control electrode control portion 423 and the power
source 424 function as the control power source 422 of FIG. 6
together.
[0048] If a means for measuring an electrification state of a local
area neighborhood (e.g., pattern portion of the semiconductor
device or peripheral area thereof) to which an electron beam is
irradiated is installed in the apparatus, an electrification state
formed by an electrification/destaticization means containing light
or electron source is measured and fed back to an
electrification/destaticization set condition, whereby an
electrification state suitable for an inspection area can be
formed. For example, when an electrification state of a
semiconductor device measured is inspected/measured, it is fed back
to a voltage (406 in FIG. 6) applied to the semiconductor device,
so that a constant electric field distribution can be always
maintained between the semiconductor device and the electrification
control electrode, and an inspection/measurement can be performed
in a set condition "as is".
[0049] A preliminary electrification/destaticization means includes
an electron source or a light source 450, a lens 451, and a control
electrode 452. An electron or light emitted from the electron
source or the light source 450 is adjusted in dispersion by a lens
to be then irradiated to the semiconductor device through the
control electrode 452. A voltage applied to the control electrode
452 is controlled by the control portion 6. An electric current
flowing through the electrode 452 may be interpreted by an
operation portion 48 if necessary, and the interpretation result
may be transmitted to the control portion 6.
[0050] The detecting portion 7 includes the detector 20 within the
vacuum-exhausted chamber 2, and the pre-amp 21, the AD converter
22, an optical converter 23, an optical fiber 24, an electrical
converter 25, a high voltage power source 26, a pre-amp driving
power source 27, an AD converter driving power source 28, and a
reverse-bias power source 29 outside the chamber 2. In the
detecting portion 7, the detector 20 is disposed above the object
lens 16 within the chamber 2. The detector 20, the pre-amp 21, the
AD converter 22, the optical converter 23, the pre-amp driving
power source 27, and the AD converter driving power source 28 are
floated to positive potential by the high voltage power source
26.
[0051] The specimen chamber 8 includes a specimen stage 30, an X
stage 31, a Y stage 32, a wafer holder 33, a position monitoring
length measuring apparatus 34, and an optical height measuring
apparatus 35. The wafer 9 is placed on the wafer holder 33.
[0052] The optical microscope portion 4 is installed at a location
adjacent to the electronic optical system 3 inside the chamber 2,
and they are apart from each other at a distance which does not
influence each other. A distance between the electronic optical
system 3 and the optical microscope portion 4 is already known.
Either the X stage 31 or the Y stage 32 reciprocates a distance
between the electronic optical system 3 and the optical microscope
portion 4. The optical microscope portion 4 includes a light source
40, an optical lens 41, and a CCD camera 42.
[0053] Operation commands and operation conditions of respective
apparatus components are inputted or outputted from the control
portion 6. The control portion 6 has a database which stores
control parameters or operation conditions of the electronic
optical system 3, the X stage 31, the Y stage 32, and so on.
Conditions--of when an electron beam is generated--such as an
acceleration voltage, an electron beam deflection width, a
deflecting speed, signal input timing of an inspection apparatus, a
specimen stage moving speed, a set of the secondary electron
converging lens are selected according to a purpose, and control of
the respective apparatus components is executed. Operations of the
respective apparatus components can be executed by a manual
operation of a user through a user interface or can be performed
according to an operation condition previously set in the control
portion 6. The control portion 6 monitors deviation of the position
or the height by signals of the position monitoring length
measuring apparatus 34 and the optical height measuring apparatus
35 by using a correction control circuit 43, generates a correction
signal based on the monitoring result, and transmits the correction
signal to the lens power source 45 or the scanning deflector 44 so
that an electron beam can be irradiated to a proper position. A
shading state is interpreted from an SEM image formed of a
secondary signal, and the information as a correction signal is
transmitted to a control portion 70 of the secondary signal
converging lens through the control portion 6 so that shading does
not occur.
[0054] In order to acquire an image of the wafer 9, a finely
concentrated electron beam 19 is irradiated to the wafer 9 to
generate either or both 51 of a secondary electron or reflected
electron, and detection is performed by synchronizing them with
scanning of the electron beam 19 and movement of the stages 31 and
32 if necessary, whereby an image on a surface of the wafer 9 is
acquired.
[0055] As the electron source 10, a thermal field emission electron
source of a common type is used. Since a more stable electron beam
current can be secured compared to a conventional electron source
such as a tungsten (W)-filament electron source or a cold field
emission electron source when the electron source 10 is used, a
potential contrast image which with a small brightness fluctuation
can be obtained. The electron beam 19 is extracted from the
electron source 10 when a voltage is applied between the electron
source 10 and the extracting electrode 11. The electron beam 19 is
accelerated when negative potential of a high voltage is applied to
the electron source 10.
[0056] As a result, the electron beam 19 is directed toward the
specimen stage 30 at energy corresponding to the electric
potential, is converged to the condenser lens 12, and is finely
concentrated by the object lens 16 again to be finally irradiated
to the wafer 9 placed on the X, Y stages 31 and 32 on the specimen
stage 30. A scanning signal generator 44 which generates a scanning
signal and a blanking signal is connected to the blanking deflector
13, and the lens power source 45 is connected to the condenser lens
12 and the object lens 16.
[0057] A negative voltage (retarding voltage Vr) can be applied to
the wafer 9 on the wafer holder 33 by the retarding power source 36
and the retarding power source control portion 68. By adjusting the
voltage from the retarding power source 36, it is possible to
accelerate a primary electron beam and to adjust energy of the
electron beam irradiated to the wafer 9 without changing electric
potential of the electron source 10.
[0058] Either or both 51 of the secondary electron and the
reflected electron generated by irradiating the electron beam 19 to
the wafer 9 are accelerated by a negative voltage applied to the
wafer 9. The secondary signal focusing lens 69 is disposed above
the wafer 9, and dispersion of either or both 51 of the secondary
electron and the reflected electron accelerated is adjusted by the
lens 69. The control portion 70 which controls the lens 69 can vary
it, in connection with an optical condition of a primary electron
beam containing a negative voltage applied to a specimen and a set
condition of the electrification control electrode 65. Also, the
E.times.B deflector 18 is disposed to deflect either or both 51 of
the secondary electron and the reflected electron accelerated in a
predetermined direction. The deflection amount can be adjusted by a
voltage and the strength of electric field applied to the E.times.B
deflector 18. The electric field can be varied, in connection with
a negative voltage applied to a specimen. Either or both 51 of the
secondary electron and the reflected electron are adjusted in
dispersion and directing direction by the lens 69 and the E.times.B
deflector 18 to thereby collide with the reflecting plate 17 under
a predetermined condition. If either or both 51 of the secondary
electron and the reflected electron accelerated collide with the
reflecting plate 17, either or both 52 of a second secondary
electron and reflected electron are generated from the reflecting
plate 17.
[0059] The second secondary electron and backscatter electron 52
generated by collision with the reflecting plate 17 are guided to
the detector 20 due to attraction electric field. The detector 20
is configured to detect either or both 52 of the second secondary
electron and reflected electron which are generated such that
either or both 51 of the secondary electron and reflected electron
generated while the electron beam 19 is being irradiated to the
wafer 9 are accelerated to collide with the reflecting plate 17, in
connection with scanning timing of the electron beam 19. An output
signal of the detector 20 is amplified by the pre-amp 21 installed
outside the chamber 2 and is converted into digital data by the AD
converter 22. The AD converter 22 is configured to immediately
convert the analog signal which is detected by the detector 20 and
then amplified by the pre-amp 21 into a digital signal to be
transmitted to the image processing portion 5. Since the analog
signal detected is digitalized directly after detected and is then
transmitted, a high speed signal with a high SN ratio can be
obtained. Here, as the detector 20, for example, a semiconductor
detector may be used.
[0060] The wafer 9 is placed on the X, Y stages 31 and 32, and an
inspect may be executed by selecting either of a method for
stopping the X, Y stages 31 and 32 and two-dimensionally scanning
the electron beam 19 and a method for continuously moving the X, Y
stages 31 and 32 in a Y direction and scanning the electron beam 19
in an X direction in a straight line form. In case of inspecting a
relatively small area, a method for stopping the stages to perform
an inspection is effective, whereas in case of inspecting a
relatively large area, a method for moving the stages to perform an
inspection is effective. If it is required to blank the electron
beam 19, the electron beam 19 is deflected by the blanking
deflector 13 so that the electron beam does not pass through the
iris 14.
[0061] As the position monitoring length measuring apparatus 34, in
the first embodiment of the present invention, a length measuring
system using laser interference is used. It is configured to
monitor positions of the X stage 31 and the Y stage 32 in real time
and to transmit them to the control portion 6. Data such as the
number of rotations of the wafer holder 33 as well as data about
the X stage 31 and the Y stage 32 are also transmitted to the
control portion 6 from the respective drivers, and the control
portion 6 is configured to accurately recognize an area or a
position to which the electron beam 19 is irradiated based on these
data, and deviation of an irradiation position of the electron beam
19 is corrected by the correction control circuit 43 in real time
if necessary. Also, an area to which an electron beam is irradiated
is memorized for each wafer.
[0062] As the optical height measuring apparatus 35, an optical
measuring apparatus of a measuring method which does not use an
electron beam, for example, a laser interference measuring
apparatus or a reflected-light type measuring apparatus, is used.
The optical height measuring apparatus 35 is configured to measure
the height of the wafers 9 placed on the X, Y stages 31 and 32 in
real time. In the first embodiment of the present invention, used
is a technique that white light emitted from the light source 37 is
irradiated to the wafer 9, a position of reflected light is
detected by a position detecting monitor, and a variation of the
height is computed based on a change of a position. A focal
distance of the object lens 16 for finely concentrating the
electron beam 19 is dynamically corrected based on measured data of
the optical height measuring apparatus 35, so that the electron
beam 19 always focused on a non-inspection area can be irradiated.
Bending or height transformation of the wafer 9 may be measured in
advance before irradiation of an electron beam, and a correction
condition of the object lens 16 may be set for each inspection area
based on the measurement data.
[0063] The image processing portion 5 includes an image storing
unit 46, a computing unit 48, and a monitor 50. In the computing
unit 48, a software for computing electrification potential on an
inspected specimen surface based on the detection result of the
detector 7 and a software for processing the detection result of
the detector 7 to perform a defect inspection of an inspected
specimen are stored, and an operation for detecting electrification
potential and an operation for a defect inspection are executed.
Even though not shown in the drawing, the monitor 50 is equipped
with an information input means through which an apparatus user
sets or inputs information necessary for a control system of an
apparatus, and a user interface is configured by the monitor 50 and
the information input means. An image signal of the wafer 9
detected by the detector 20 is amplified by the pre-amp 21, is
digitalized by the AD converter 22, is converted into an optical
signal by the optical converter 23, is transmitted through the
optical fiber 24, is converted into an electrical signal by the
electrical converter 25, and is stored in the image storing unit
46.
[0064] An irradiation condition of an electron beam and various
detection conditions of the detection system to form an image are
set in advance, and are stored in the database in a file form.
[0065] Next, in the apparatus configuration according to the first
embodiment of the present invention, a sequence for
inspecting/measuring a semiconductor device will be described with
reference to FIG. 2. A semiconductor device is carried into the
specimen chamber 8 through the specimen exchange chamber 62 (200).
Then, the retarding voltage Vr and an initial voltage are applied
to the electrification control electrodes A and B (201) to
pre-charge or destaticize a required area (202). Electrification of
a pattern area which is an inspection target and a peripheral area
thereof is measured (203), and pre-charging or destaticizing is
repetitively performed until they becomes an optimum
electrification state necessary for an inspection/measurement
(204). After pre-charging/destaticizing is finished, it is
transferred to a present inspection phase (205), it is determined
whether to charge a semiconductor device or not before or after
inspection/measurement image acquisition by using an electron beam
for an inspection/measurement (206), a charging setting is
performed if necessary (207), an electronic optical system for an
inspection/measurement is set (208) to perform a
measurement/inspection (209), and the sequence is finished (210).
The reason for performing a setting containing each electrode in an
inspection/measurement is to use an optimum value for an
inspection/measurement corresponding to an electrification state
after pre-charging/destaticizing.
[0066] In order to pre-charge or destaticize a semiconductor
device, besides a primary electron beam for an
inspection/measurement, a pre-charge/destaticizing light
source/electron source may be used.
[0067] Next, a setting of each electrode in an
inspection/measurement will be described centering on a case where
a semiconductor device is charged to a negative by pre-charging and
an a non-conduction fault inspection of a hole pattern is
performed. In order to maintain an electrification state of a
semiconductor device during an inspection/measurement, it is
necessary to set a voltage between a pattern area which is an
inspection target and an electrification control electrode to a
small value during an inspection/measurement (<10 V).
Electrification of a semiconductor device by pre-charging is
measured, and the electrification voltage is absorbed by adjusting
at least of the electrification control electrode B421 and the
retarding voltage Vr. A set voltage between the electrification
control electrode B421 and a semiconductor device surface during an
inspection/measurement depends on an electrical characteristic of a
semiconductor device and thus is determined by an empirical value
or a previous investigation result.
[0068] In detail, based on a correlation between a difference
between a set potential of the electrification control electrode
B421 and the retarding potential Vr, and electrification potential
of an inspection area of a semiconductor device after an
inspection, at least one of the electrification control electrode
B421 and the retarding voltage Vr is adjusted, and a setting is
performed so that a surface potential difference between the
electrification control electrode and a semiconductor device can be
equal to a potential difference between the two after an
inspection. As a result, it is possible to suppress the secondary
electrons 428 which return to a semiconductor surface due to an
off-axis potential barrier shown in FIG. 5A and to prevent
excessive electrification of a semiconductor device during an
inspection/measurement. A set voltage of the electrification
control electrode B is set by 66 so that a contrast of a defective
portion can be highest.
[0069] As described above, using the apparatus according to the
first embodiment of the present invention, since surface potential
of a semiconductor device becomes almost equal to electric
potential of the electrification control electrode B421 in the
above-described method, a secondary signal emitted from a
semiconductor device by irradiation of a primary electron beam can
maintain a constant electrification state during an
inspection/measurement without charging an area beyond a scanning
range of a primary electron beam. Also, an inspection/measurement
of high sensitivity and high stability (reproducibility) can be
realized by optimizing a set voltage of the electrification control
electrode A420.
Second Embodiment
[0070] A second embodiment of the present invention will be
described centering on an example that in order to form an optimal
electrification state for an inspection target on a semiconductor
device by pre-charging, a correlation between set potential of
pre-charging and electrification of a pattern portion and a
peripheral insulating layer after pre-charging and destaticizing is
measured in advance, and pre-charging is performed based on the
information before an inspection/measurement. The second embodiment
of the present invention uses the same apparatus configuration as
that of the first embodiment of the present invention, and thus
only a setting method and an inspection/measurement sequence using
the correlation information will be described.
[0071] FIG. 3A shows a measurement result according to the second
embodiment of the present invention. A semiconductor device is
pre-charged or destaticized by applying a set voltage for
pre-charging/destaticizing (at least one of the retarding voltage
Vr set by the control portion 68, a set voltage of the
electrification control electrode A420 set by the control portion
66, a set voltage of the electrification control voltage B421
controlled by the control portion 423, and a set voltage of the
control electrode 452 for preliminary
electrification/destaticization). Then, electrification of each
position on a semiconductor device surface is measured, and a
distribution correlation between a set voltage for
pre-charging/destaticizing and an actual electrification voltage is
measured, and the data (FIG. 3B) is stored in the apparatus control
portion 6 in advance.
[0072] FIG. 7 shows an inspection/measurement sequence using the
correlation data according to the second embodiment of the present
invention. After a semiconductor device (wafer) is carried into a
specimen chamber (700), a range for pre-charging a semiconductor
device and electrification potential are inputted through a graphic
user interface (GUI) (701). A set value of each electrode for
pre-charging/destaticizing is read out from the database of FIG. 3B
(702), pre-charging/destaticizing is performed (703),
electrification of a pattern portion is confirmed (704), a setting
of an optical system and an inspection/measurement are performed
(705 and 706), and the inspection sequence is finished (707).
Third Embodiment
[0073] In order to perform an optimum setting in an
inspection/measurement, it is necessary to measure an
electrification state of an inspection area and to feed it back to
a setting, and electrification potential may be measured by using a
primary electron beam. A third embodiment of the present invention
will be described centering on a method for measuring
electrification potential using a primary electron beam.
[0074] FIGS. 8A and 8B show a principle and a sequence for
measuring electrification potential of a semiconductor device using
a primary electron beam 19 (see FIG. 1), respectively. As shown in
FIG. 8A, the principle is that an electronic signal
strength-retarding voltage curve is obtained by measuring the
electronic signal strength obtained by a detector while applying a
retarding voltage. Since a retarding voltage is lower than an
initial acceleration voltage of a primary electron beam at an
initial stage, a primary electron beam is not incident to a
semiconductor device and is reversed above a semiconductor device
to be detected by a detector ((1) of FIG. 8A). As a retarding
voltage is more shifted in a forward direction, a position that a
primary electron beam is reversed is closer to a semiconductor ((2)
of FIG. 8A). If a retarding voltage is shifted in a forward
direction so that a primary electron beam contacts a semiconductor
device surface, a secondary signal is generated from a surface and
detected by a detector ((3) of FIG. 8A). The electronic signal
strength detected by a detector when a primary electron beam
contacts a semiconductor device surface has curves 460 and 461
shown in (4) of FIG. 8A since the strength that a primary electron
beam is detected is different. The curve 460 is obtained from a
specimen that electrification is already known, for example, it is
measured from a holder around a wafer or a standard specimen (no
electrification). The curve 461 is a curve measured from a
measurement target. Electrification potential of a measurement
target can be obtained from shift of the two.
[0075] FIG. 8B shows a measurement sequence. An optical condition
for potential measurement is imported in advance (800). Then, it
moves to an initial measurement position of a primary electron beam
(801), and a brightness signal is extracted by acquiring an SEM
image (802). Next, a retarding voltage is changed in a forward
direction (804), and a brightness signal is extracted by acquiring
an SEM image again. When an SEM image is acquired, a variation of
electrification on a semiconductor device surface due to
irradiation of a primary electron beam is suppressed, and in order
to more accurately measure than in an original electrification
state, it is effective to make an acquisition position of an SEM
image (805) deviated whenever a retarding voltage is changed. An
electron signal strength-retarding voltage curve can be obtained by
repetitively performing the above-described operation until a
maximum value is obtained. The standardization for the acquired
curve is performed (806), it is compared to a reference curve
(curve 406 shown in FIG. 8A), electrification potential of an
inspection target is obtained (807), and the sequence is completed
(808).
[0076] Measurement according to the third embodiment of the present
invention is performed at plural positions on a semiconductor
device surface, and thus it is possible to investigate an
electrification potential distribution on a semiconductor device
surface which is an inspection/measurement target, and it is also
possible to inspect/measure a semiconductor device always in an
optimum condition by feeding the information back to an
inspection/measurement condition.
[0077] According to the present invention described above in
detail, since a means for measuring electrification potential of a
semiconductor device which is an inspection/measurement target is
provided, it is possible to optimize pre-charging or to optimize a
set voltage of each electrode containing an electrification control
electrode A in an inspection/measurement. Also, by feeding the
measurement result to a set voltage of an electrification control
electrode B or a retarding voltage, it is possible to have an
inspection area to maintain a constant electrification state during
an inspection/measurement, whereby an inspection/measurement of
high sensitivity and high reproducibility can be realized. In the
above description, an electron beam is used as a charged particle
beam; however, the present invention can be applied to a technique
which performs an inspection/measurement by using a charged
particle beam of a different kind such as an ion beam.
* * * * *